1. Field of the Invention
This invention relates to a magneto-optical disc used as a recording medium for an external memory of an electronic computer or a recording/reproducing apparatus adapted for recording/reproducing audio or video signals.
2. Description of the Prior Art
There has hitherto been proposed a magneto-optical disc having a signal recording layer with which perpendicular recording is possible. For writing information signals on the magneto-optical disc, the region of the signal recording layer in which to write the information signals is heated to not lower than the Curie temperature by converging and radiating a laser beam thereto. Simultaneously, an external magnetic field is applied to the region. The signal recording layer, which has been heated to at least the Curie temperature, is substantially bereft of coercivity, so that its direction of magnetization is inverted so as to follow the direction of the applied external magnetic field. By inverting the direction of magnetization of the signal recording layer in this manner, it becomes possible to write information signals in the form of, for example, digital signals.
Meanwhile, the magneto-optical disc is usually loaded alone on the recording apparatus for recording information signals, similarly to the optical disc, such as the compact disc. Since the base plate of the magneto-optical disc, on which the signal recording layer is formed, is constituted by a synthetic resin, such as transparent polycarbonate resin, warping tends to be produced in the base plate due to heat shrinkage during molding, as a result of which the disc surface tends to undergo deviations from the horizontal during rotational driving of the disc. If the disc surface undergoes deviations from the horizontal during disc rotation, the distance between the magnetic head impressing the magnetic field and the signal recording surface of the disc is changed. Thus the strength of the magnetic field impressed by the magnetic head on the signal recording layer is fluctuated, so that optimum recording of the information signals cannot be achieved. If the deviations of the disc surface during disc rotation be excessive, destruction may occur if the magnetic head is brought into contact with the recording layer of the magneto optical disc.
For preventing incorrect recording due to disc surface deviations or damage to the signal recording surface of the disc, as well as to prevent wear or damage to the magnetic head, there has also been proposed a magneto-optical disc in which a disc-shaped lubricious sheet of a reduced thickness, formed of a high polymeric material, such as polyethylene terephthalate, is provided on the recording and/or reproducing apparatus and adapted to be brought, into intimate contact with the surface of a magneto-optical disc facing a contact type magnetic head during recording of the information signals, so that the magnetic head is thrust into sliding contact with the lubricious sheet. With the use of such lubricious sheet, the magnetic head may be slidingly contacted with the lubricious sheet under a predetermined constant force, even though the magneto-optical disc surface undergoes deviations from the horizontal, so that a predetermined constant distance may be maintained between the magnetic head and the magneto-optical disc to provide a uniform magnetic field impressed on the magneto-optical disc.
However, since the magneto-optical disc is loaded alone in the recording and/or reproducing apparatus, foreign matter, such as dust and dirt from an external environment, tends to be deposited on the disc surface. If the lubricious sheet is provided on the recording/reproducing apparatus, the distance between the magnetic head and the signal recording layer of the disc tends to be changed by dust and dirt occasionally affixed between the lubricious sheet and the recording layer side of the disc. As a result, the magnetic field impressed from the magnetic head to the signal recording layer tends to be changed in magnitude, thereby producing the effect of amplitude modulation.
For overcoming such drawback, it has also been contemplated that the magneto-optical disc be accommodated in a cartridge composed of an Upper half and a lower half abutted and connected to each other as a disc Cartridge similarly to a resilient fragile floppy disc. However, if such disc cartridge is used as a recording medium, the magnetic head cannot be slidingly contacted with the magneto-optical disc accommodated in the cartridge, so that recording of information signals cannot be made because as the cartridge is completely hidden by the lubricious sheet of a size large enough to cover the magneto-optical disc provided on the recording/reproducing apparatus to render it impossible to cause the magnetic head to be intruded into the inside of the cartridge.